test/net/imap/test_imap.rb: use Thread#stop? to wait for server thread

Still timeout occurs.  Retry for 5dd8fdd3f3.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11x/ruby-master/log/20190625T032405Z.fail.html.gz

test/net/imap/test_imap.rb

@@ -130,12 +130,11 @@ class IMAPTest < Test::Unit::TestCase
   end
 
   def start_server
-    started = false
+    th = Thread.new do
-    @threads << Thread.new do
-      started = true
       yield
     end
-    sleep 0.1 until started
+    @threads << th
+    sleep 0.1 until th.stop?
   end
 
   def test_unexpected_eof